Image of the day

Captured by
Troy Carpenter

2024 MAY 9 - Giant Sunspot AR3664

My Account

New to Astromart?

Register an account...

Need Help?

Posts Made By: Preston Engebretson

March 14, 2004 04:46 PM Forum: Telescope Making

Mirror maker Steve Kennedy's web address???

Posted By Preston Engebretson

I've done a search and come up with nothing useful. I believe that he makes mirrors in the 20-32" range.

Any help would be appreciated.

Thanks Preston

April 1, 2004 09:59 PM Forum: Telescope Making


Posted By Preston Engebretson

A sphereometer about 20-33% of the mirrors diameter is nice to be able to check that the surface is uniform and one is not grinding astigmatism into the mirror.

Enco has dial indicators that measure .0001" with a travel of 0.100 " for $50-60.00 That and a base with 3 holes drilled equidistant around a hole in the center for the dial indicator, and then epoxy ball bearings into each of the 3 holes for little feet.

Something like this will provide more than enough accuracy for all but the most demanding work.

Hope that helps, Preston

December 9, 2004 04:02 AM Forum: Telescope Making

Richard D Shaffer's where abouts???He's Missing?

Posted By Preston Engebretson

Richard was last seen posting on or about 22 November 2004 on Astromart forums...

I was last in contact with him on 15 November...regarding the purchase of some bearings that he had listed...

Does anyone know what has become of Richard???

Thanks and Best Regards,


January 22, 2005 01:11 AM Forum: Telescope Making

Richard Shaffer is alive and I have my bearings

Posted By Preston Engebretson

Though I have not heard from Richard since Thanksgiving and was about to call him again to see how he was doing...

This evening my wife said...Preston, there is a box from Richard Shaffer in Arizona at the front door...

Bearing as discribed and packed very well...

Thank You Richard and Hope your Feeling Better,

Very Best Regards,


June 28, 2005 07:30 PM Forum: Telescope Making

Re: Core drilling a mirror

Posted By Preston Engebretson

Hi Vic,

Most places where one buys the glass will preform that service.

A few are named below:

Glass Fab Inc.
257 Ormond St, Rochester, NY 14605, (716) 262-4000.

Newport Glass Works, Ltd.
1631 Monrovia Avenue, Costa Mesa, CA 92627 Tel: (949)645-1500, (800)784-2100

United Lens Co., Inc.
259 Worcester St. Southbridge, MA 01550-1325, 508/765-5421 Fax: 508/765-0500

I'd personally recommend either Glass Fab or United Lens...Had issues with the other on a very poor anneal and shipping orders in a timely mannor.



August 23, 2005 10:08 PM Forum: Pictures of Me and My Telescope and........

Baby has a new Home on the George 36"RC

Posted By Preston Engebretson

Hello Everyone,
Well rather than try to make a mount and the little monster portable, we've spent the past month machining a mounting system and aquiring permission to mount her on something very fitting.

Lenses were figured by Roger Ceragioli and are said to be one of the finest Fraunhofer 11" f15 doublets around...time will tell.

The OTA is 165" long and with cell in front weighs about 150 pounds...thus we were lucky to just get the OTA balanced yesterday. Hopefully well get everything else balanced for first light during the up coming new moon.

Hope you enjoy the picture taken yesterday.



January 23, 2006 04:17 PM Forum: Home Observatories

Re: Schmidt-Newt and Exposure to the Elements

Posted By Preston Engebretson

Hi Jeff, I live in Houston and am also building an observatory.

I'm on the Observatory Committee of the Houston Astronomiocal Society with our observatory in Columbus and am affiliated with the George Observatory with the 36 inch RC and 11 inch refractor.

The key to keeping optics in good condition is to us something that dehumidifies the air. In Columbus we have a C14 and two 12.5 inch refractors in a building that has a sliding roof. There we have a dehumidifier and airconditioner in the scope area. The airconditioner is set to 78 degrees and the DH to run if the Humidity rises above 50% RH.

At the George it is not effecient to run a true DH unit at the dome as it is 50+ feet in diameter. There they use a heating unit the measures the outside air temp and inside temp. The unit as I understand keeps the air inside 2 degrees+ above the outside temp...during the fall, winter and spring...not sure how they handle the summer.

My dome is 10 feet inside diameter and I plan to a small DH unit and a chiller from our ground water pump( temp of water 52 deg F) so that I can control the inside temp to the expected night temps for the early evening. The dome will house a 14 inch Lurie-Houghton and I would not want the corrector to dew up or mold...

One can also cover the optics, but in my case the entire dome and scope will be robotic and having to uncover the optic end would add even more complexity.

Another possibility for correctors is running a dew control system that sences the corrector temp versas air temp and keeps the corrector at a couple of degrees above air temp...

Hope that has helped,



January 14, 2003 02:50 PM Forum: Telescope Making

Need some help

Posted By Preston Engebretson

Hello Marty,

It's highly likely that the fracture will grow. Changes in temperarure of the mirror will provide the needed expansion and contraction, that will cause further stresses and growth of the current fracture.

Look at a car windshield that gets cracked by a rock. It's the temperature changes more than anything else that cause the cracks growth.

Clear Skys,


January 30, 2003 02:04 PM Forum: Telescope Making

Best wood for tripod legs?

Posted By Preston Engebretson

Hello Victor,

From what I've read, Black Walnut...the straightest grain you can find...offers the highest dampening as far as wood goes.

I've got a beautiful 10" RC mounted on stressed crutch style
Black Walnut legs and it shows very quick dampening of any accidental vibrations caused by touching any part of the scope.

Each of the 3 legs is composed of 2 seperate black walnut tapered parts as in a crutch that are seperated by spacers made of aluminum.

If you'd like, I could send you a picture. It would be clearer for your understanding I think.



January 31, 2003 02:09 AM Forum: Telescope Making

Best wood for tripod legs?

Posted By Preston Engebretson

Hi Victor,

As a side note...Do you think that Takahashi would use Black Walnut if it were not the best for the purpose...

It's not grown in Japan, and is imported and VERY expensive there...

Also look at the engineering numbers for vibration dampening, It's head and shoulders above the others...

Perhaps that's why the best use it and only it....